title: A cytosolic STIM2 preprotein created by signal peptide inefficiency activates ORAI1 in a store-independent manner.
abstract: Calcium (Ca(2+)) influx through the plasma membrane store-operated Ca(2+) channel ORAI1 is controlled by Ca(2+) sensors of the stromal interaction molecule (STIM) family. STIM1 responds to endoplasmic reticulum (ER) Ca(2+) store depletion by redistributing and activating ORAI1 from regions of the ER juxtaposed to the plasma membrane. Unlike STIM1, STIM2 can regulate ORAI1 in a store-dependent and store-independent manner, but the mechanism by which this is achieved is unknown. Here we find that STIM2 is translated from a highly conserved methionine residue and is directed to the ER by an incredibly long 101-amino acid signal peptide. We find that although the majority of the total STIM2 population resides on the ER membrane, a second population escapes ER targeting to accumulate as a full-length preprotein in the cytosol, signal peptide intact. Unlike STIM2, preSTIM2 localizes to the inner leaflet of the plasma membrane where it interacts with ORAI1 to regulate basal Ca(2+) concentration and Ca(2+)-dependent gene transcription in a store-independent manner. Furthermore, a third protein comprising a fragment of the STIM2 signal peptide is released from the ER membrane into the cytosol where it regulates gene transcription in a Ca(2+)-independent manner. This study establishes a new model for STIM2-mediated regulation of ORAI1 in which two distinct proteins, STIM2 and preSTIM2, control store-dependent and store-independent modes of ORAI1 activation.
authors.affiliation: School of Biological Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Sarah J L
authors.initials: SJ
authors.lastname: Graham
authors.forename: Marie A
authors.initials: MA
authors.lastname: Dziadek
authors.forename: Lorna S
authors.initials: LS
authors.lastname: Johnstone
mesh_terms: D001483:Base Sequence; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000235:genetics / Q000378:metabolism*; D015815:Cell Adhesion Molecules / Q000235:genetics / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D008954:Models, Biological; D008969:Molecular Sequence Data; D000071740:ORAI1 Protein; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism*; D021382:Protein Sorting Signals / Q000502:physiology*; D021381:Protein Transport / Q000502:physiology; D000071738:Stromal Interaction Molecule 2
chemical_list: D015220:Calcium Channels; D015815:Cell Adhesion Molecules; D000071740:ORAI1 Protein; C510746:ORAI1 protein, human; D011498:Protein Precursors; D021382:Protein Sorting Signals; C513279:STIM2 protein, human; D000071738:Stromal Interaction Molecule 2; D002118:Calcium